Project Summary. Hoarding disorder (HD) is a severe psychiatric disorder that affects approximately 3-6% of the population8,27. HD poses an enormous public burden, as it is associated with high rates of unemployment and utilization of public aid programs and healthcare services, and extreme clutter often results in fire hazards, higher rates of evictions, and in some cases, death9,10. Despite this impact, treatment for HD is only moderately effective, which points to a need to identify factors that may not be sufficiently targeted in existing treatments11. One of the core features of HD is a maladaptive attachment to possessions12,13, and theorists suggest individuals may attempt to compensate for poor interpersonal connections through connections to objects21,22. Though attachment has often been conceptualized within an interpersonal framework, the core processes of interpersonal attachment may provide important information regarding attachment to non-human entities, such as objects. According to the Research Domain Criteria put forth by the National Institute of Mental Health, attachment is measured as the tendency to show preference for a particular person for comfort, as well as increased distress upon separation from that person1. From a biological perspective, oxytocin and sympathetic arousal may underlie attachment difficulties as oxytocin decreases and sympathetic arousal increases when individuals are separated from attachment figures2,3,5. Taken together, it may be that individuals with HD exhibit differential patterns of biobehavioral responses to people and possessions that would indicate dysfunction of the attachment system, which may help to explain their reliance on and extreme difficulty parting with possessions. Importantly, elucidating the nature and mechanisms of this relationship is imperative in order to advance our understanding of and treatment for HD. The proposed study will delineate biobehavioral mechanisms underlying the relationship between hoarding and attachment. Specifically, this study will: a) investigate differences in self-report attachment to possessions and people in individuals with and without HD, b) compare biological and emotional responses associated with behavioral manifestations of attachment (i.e., preference and distress upon separation) to possessions in individuals with and without HD, and c) compare indicators of attachment to both possessions and people in individuals with and without HD through two expressive writing tasks. The experience gained through this fellowship will prepare the applicant for an independent research career investigating transdiagnostic risk factors of anxiety-related psychopathology using a translational approach.